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Planar versus Nonplanar Pd Clusters: Stability and CO Oxidation Activity of Pd Clusters with and without TiO₂(110) Substrate

Li, Yanle, Liu, Chunyan, Pu, Zhichen, Bao, Linquan, Zhu, Yan, Ma, Jing
Journal of physical chemistry 2019 v.123 no.22 pp. 13739-13747
X-ray photoelectron spectroscopy, activation energy, adsorption, algorithms, atomization, carbon monoxide, catalytic activity, cluster analysis, density functional theory, energy, magnetism, nanoparticles, oxidation, oxygen, palladium, titanium dioxide
Metal clusters with different sizes and shapes may have very different optical, electronic, magnetic, and catalytic properties. The possible geometrical structures of Pdₘ (m = 3–12, the number of Pd atoms) have been explored by the combination of a modified Morse potential driven genetic algorithm search with density functional theory (DFT) calculations. By using the clustering analysis of DFT data, the studied Pdₘ clusters were grouped into two families of m > 8 (with longer average interatomic distance and larger atomization energy) and m < 8, respectively. The Pd₈ cluster just stands in between these two groups. The relative stability, adsorption energy of CO and O₂, and CO oxidation activity of planar (2D) and nonplanar (3D) Pd₈ clusters with and without TiO₂(110) substrate have been studied theoretically and experimentally, in comparison with the catalytic behavior of low-dimentional Pd₁ and Pd₃ clusters. The adsorption of CO does not largely affect structures of supported and unsupported 3D Pd₈ clusters. But the simultaneous adsorption of CO and O₂ could alter the CO oxidation activity of supported Pd₈ clusters. The barriers of the rate-determining steps of CO oxidation on the supported low-dimensional Pd₁, Pd₃, and 2D Pd₈ clusters were lower than that of the supported 3D Pd₈ clusters. This was supported experimentally by the lower activation energy of CO oxidation on the supported Pd₃ cluster (Pd₃/TiO₂) than that on the supported 3D Pd nanoparticles (Pd NPs/TiO₂). The X-ray photoelectron spectroscopy of the Pd₃/TiO₂ and Pd NPs/TiO₂ indicates that the interfacial charge transfers from the Pd cluster to the TiO₂ substrates, which is in agreement with the calculation results.